Why not both? It is always a good idea to keep some bitcoin. For one thing, it is convenient for trading to have some in reserve. Also if you hold bitcoin this gives you an incentive to keep up with bitcoin news, which obviously can affect the whole market. It is also less volatile than most coins.
Ethereum is great as well, smart contracts are going to be very important in the future.

Both are likely to grow over the next few years, so keep both (plus other alts as you see fit).